The Orion [C ii] survey also provides a way to link the observed heating efficiency directly to the characteristics of the local PAH family. [1]} and [2]} have analyzed a set of Spitzer/IRS spectra that systematically explored the behavior with distance of PAHs in the PDR associated with the Veil Shell. As Fig. REF  shows, the intensity of the [C ii], FIR, and PAH bands all smoothly decrease with distance from the Orion Bar (and hence the illuminating star). The gas heating efficiency, however, shows a clear peak at about 10 from the Bar. In contrast, the ratio of the IRAC 8 \(\mu \) m band – dominated by the 7.7 \(\mu \) m PAH feature – to the IRS 11.2 \(\mu \) m band – a blend of the PAH 11.0 \(\mu \) m and 11.25 \(\mu \) m features – is largely constant over the full distance range probed. This band ratio is generally thought to probe the degree of ionization of the emitting PAHs as the former band traces emission by ionized PAHs while the latter is dominated by neutral PAHs [3]}, [2]}. Given the general relation between \(G_0\)  and \(n\)  discussed in Sec. REF , we expect little variation in the ionization parameter with distance and that is borne out by the constant \(8\,\mu \mathrm {m}\) /\(11.2\,\mu \mathrm {m}\)  band ratio. As a corollary, this again suggests that there is another factor that affects the gas heating efficiency besides ionization. That could be abundance of PAHs and VSGs. However, the intensity ratio (\(I_{8\,\mu \mathrm {m}}+I_{11.2\,\mu \mathrm {m}})/I_{\mathrm {FIR}}\)  – a good proxy for the PAH abundance [5]}– is constant along the line cut, which hints that VSGs play an important role.
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